Biaxially stretched polypropylene films are excellent in transparency, mechanical properties, electrical characteristics, and others to be used for various purposes/articles such as packaging purposes, tapes, and electrical purposes/articles, typical examples thereof including cable wrapping purposes, and capacitors.
With respect to the use for capacitors among such various uses, the films are particularly preferably utilized not only for direct current and AC capacitors but also for high-voltage capacitors because of their excellent withstand voltage characteristics, and low loss characteristics.
Recently, various electrical facilities have been made to have an inverter function. Following this tendency, capacitors have been increasingly required to be made smaller in size and larger in capacitance. In response to such requirements of the market, it is essential for the film to be made even more thinner while a biaxially stretched polypropylene film is improved in withstand voltage characteristics and processability into elements.
In such a biaxially stretched polypropylene film, the surfaces thereof need to be appropriately roughened from the standpoint of withstand voltage characteristics and processability into elements. This is particularly important for improving the slipping property or the oil impregnation property of the film and, when the film is made into a metallized capacitor, providing preservability. In the case of an oil-impregnated type capacitor in particularly, characteristics such as the withstand voltage characteristics and the preservability are significantly impaired when the capacitor is poor in impregnation property. Thus, it is important to roughen the surfaces appropriately. The preservability herein is a function of a metallized capacitor in which a metallized film formed on the dielectric film is used as an electrode, and is a function that the vapor-deposited metal is scattered by discharge energy when an abnormal discharge is caused, to cause the capacitor to recover electrical insulation, so that a short circuit is prevented to maintain functions of the capacitor or prevent the capacitor from being broken down. This function is very useful function also from the viewpoint of security.
Examples of the method for roughening the surfaces that have ever been proposed include mechanical methods such as an embossing method and a sandblasting method, chemical methods such as chemical etching using an solvent, a method of stretching a sheet in which a polymer different from polypropylene such as polyethylene is mixed, and a method of stretching a sheet in which β-crystals are formed (see, for example, JP 51-63500 A and JP 2001-324607 A).
However, the mechanical methods and chemical methods result in a low roughness density, and the method of stretching a sheet in which β-crystals are formed is likely to form bulky or coarse projections. Thus, the resultant sheet is not necessarily sufficient in terms of roughness density, coarse projection occurrence, and the number of projections. In a film with surfaces roughened by any of these methods, gaps between layers of the films tend to be not sufficiently impregnated with oil when the film is made into a capacitor. Thus, the capacitor partially may have a non-impregnated region with ease so that the lifespan of the capacitor may be shortened. The method of stretching a sheet in which a different polymer such as polyethylene is blended has a problem of being poor in recyclability. Specifically, although few bubbles are left when a capacitor is formed, the different polymer can adversely affect when the film is recycled.
The biaxially oriented polypropylene film based on any one of these methods is insufficient in preservability when a use condition of the capacitor is a severe condition that the electrical potential gradient used is 200 V/μm or more. Thus, a problem may be caused from the standpoint of reliability. The electrical potential gradient herein denotes a property obtained by dividing the voltage applied to the dielectric film by the thickness of the film, that is, the applied voltage per unit thickness of the film.
With respect to the roughness density and the uniformity of projections, suggested are: a high melt strength polypropylene film (see, for example, JP 2001-72778 A); a laminate obtained by laminating such a high melt strength polypropylene film onto an ordinary polypropylene film (see, for example, JP 2001-129944 A); and others. However, when high melt strength polypropylene resin itself is used for a capacitor, the capacitor cannot gain sufficient heat resistance and withstand voltage characteristics because of the structure of the resin. Thus, the capacitor has a problem of being remarkably lowered in dielectric breakdown voltage, particularly at high temperatures. Moreover, the above-mentioned technique of laminating a high melt strength polypropylene resin makes it very difficult to achieve a structure having a uniform laminate thickness. Thus, in the actual circumstances, the uniformity of the laminate is damaged so that a practically satisfactory dielectric film is not obtained. Japanese Patent No. 3508515 discloses a biaxially stretched polypropylene film the surfaces of which are controlled in roughness degree, and a method for producing the film. However, both the surfaces of the film are insufficiently controlled in roughness degree and such controlling is difficult.
It could be helpful to provide a biaxially stretched polypropylene film for a capacitor that exhibits excellent withstand voltage characteristics and reliability particularly when the film is used for a capacitor of AC voltage, and that ensures a stable processability into elements.